The invention lies in the field of semiconductors. The invention relates to an apparatus and a method for imaging and scanning masks for semiconductor production.
Such photomasks are needed for the exposure steps in semiconductor production. In order, for example, to be able to implement structure sizes of 180 nm in a CMOS process, photomasks with structure sizes of about 700 nm (that is to say about four times the structure size to be produced) are needed. The exposure of the semiconductor substrate coated with photosensitive lacquer is then carried out through such photomasks. Typically, etching and evaporation steps then follow the exposure step.
For the purpose of quality control in the production of such photomasks, because of the typical structure size of 700 nm, optical microscopy is the clear choice. A photomask includes a glass substrate, onto which the desired structure is applied by evaporating molybdenum silicide (MoSi) or chromium. The transparent and opaque areas of the photomask can be represented with good contrast by using optical microscopy methods.
In addition to optical microscopy, scanning methods such as scanning probe microscopy are gaining significance in the analysis of photomasks. In the case of such methods, a probe scans the height profile of the mask surface. Piezoceramic actuators permit the highly accurate positioning of the probe relative to the mask surface so that the height relief on the layer side of the mask can be registered with high resolution. In the use of scanning probe microscopy, a problem arises relating to placing the scanning probe at the correct point on the mask surface at the start of the scanning operation. In the case of manually position of the probe on the mask, it is often not clear which area of the surface is currently being scanned. Therefore, it would be helpful to be able to position the probe with a view of the mask.
Prior art includes equipping a scanning probe microscope or a surface profilometer with imaging optics and a miniature camera for enabling a track of the position of the probe on the mask surface. The drawback with such a solution is, first, the small field of view of the camera. Second, in such an embodiment of the scanning probe microscope or the surface profilometer, a significant number of functions have to be integrated in an extremely small space, which makes the handling of the scanning instrument more difficult. It is difficult to carry out adjustment work and to replace damaged scanning probes.
It is accordingly an object of the invention to provide a device and method for combining scanning and imaging methods in checking photomasks that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that makes the scanning and imaging of defined areas of the mask surface easier.
With the foregoing and other objects in view, there is provided, in accordance with the invention, an apparatus for imaging and scanning masks for semiconductor production, the masks having a layer side and an opposing side, the apparatus including a scanning instrument having a probe, the probe disposed to scan a layer side of a mask, an optical microscope for imaging a detail of the mask from a side of the mask opposing the layer side, the optical microscope disposed opposite the scanning instrument with respect to the mask, a positioning device positioning the scanning instrument and the optical microscope relative to one another such that the optical microscope images the probe and the mask simultaneously, and a displacing device connected to the mask and laterally displacing the mask between and relative to the scanning instrument and the optical microscope to permit selection,of an imaged detail of the mask while a relative position between the scanning instrument and the optical microscope is maintained.
In the case of the apparatus according to the invention, the scanning of the mask is carried out from the layer side, while the optical microscope images the mask structures from the side of the mask facing away from the layer side. By using such an inverted microscope, the upper side of the mask can be scanned while the optical imaging is being carried out from the underside of the mask at the same time. As such, the scanning instrument and the optical microscope are prevented from hampering each other. The two instruments can be positioned freely relative to the mask and relative to the respective other instrument.
In addition, the apparatus according to the invention has a positioning device or means for positioning the scanning instrument and the opposing optical microscope, with which the two instruments can be positioned relative to each other such that the probe of the scanning instrument can be imaged by the optical microscope at the same time. As soon as the probe of the scanning instrument can be detected in the microscope image, the two instruments are in mutually opposite positions, which are connected by an axis running at right angles to the mask surface (so-called xe2x80x9con-axisxe2x80x9d position). In such a position, the scanning instrument is fixed relative to the optical microscope.
In the solution according to the invention, the probe is positioned on the mask by lateral displacement of the mask relative to the two instruments fixed in the relation to each other. For such a purpose, the apparatus according to the invention includes a displacing device or means for the lateral displacement of the mask. With the displacing device, the imaged detail of the mask can be selected while maintaining the relative position between the scanning instrument and the opposing optical microscope. Because the relative position between the scanning instrument and the opposing microscope has been fixed once for all in the xe2x80x9con-axisxe2x80x9d position, the position of the probe in the field of the view of the optical microscope remains unchanged. The probe is, therefore, continuously visible in the image supplied by the optical microscope, even when the position of the mask surface is changed relative to the probe.
As soon as the two instruments have been fixed once in the xe2x80x9con-axisxe2x80x9d position in relation to each other, various areas of the photomask can be moved to and examined. Thus, the time-consuming and tiresome search for the probe in the field of view of the optical microscope is eliminated. Such elimination makes possible rapid and precise positioning of the probe on the mask surface. Various critical production structures on the photomask can be moved to one after another and then respectively be imaged and scanned.
With the objects of the invention in view, there is also provided a method of imaging and scanning masks for semiconductor production including the steps of placing a scanning instrument having a probe at a position to scan a layer side of a mask with the probe, placing an optical microscope on a side of the mask opposite the layer side at a position to image a detail of the mask from the side of the mask opposite the layer side, positioning the scanning instrument and the optical microscope relative to each other such that the optical microscope images the probe and the mask at the same time, laterally displacing the mask while maintaining a relative position between the scanning instrument and the optical microscope, and at least one of imaging and scanning a selected detail of the mask.
The method according to the invention for imaging and scanning masks for semiconductor production includes three steps. In a first step, the scanning instrument and the optical microscope are positioned relative to each other. The positioning is carried out such that the probe of the scanning instrument can be imaged by the optical microscope at the same time. The position of the probe of the scanning instrument can, therefore, be detected from the image of the mask supplied by the optical microscope. Such. image shows that the scanning instrument and the opposing optical microscope are located in the xe2x80x9con-axisxe2x80x9d position. Then, in a second step, the mask is displaced laterally, the relative position between the scanning instrument and the opposing optical microscope being maintained. Therefore, the probe of the scanning instrument can be positioned as desired relative to the photomask. In the process, the probe always remains detectable in the image supplied by the optical microscope. When the desired detail of the mask has been found, the selected mask area is imaged and/or scanned in a third step.
The advantage of the method according to the invention is that the tiresome and time-consuming search for the probe of the scanning instrument only has to be carried out just once for each mask examined.
In accordance with another feature of the invention, the displacing device for laterally displacing the mask includes a displaceably mounted mask table. The photomask is fixed on the mask table and then can be moved in two coordinate directions at right angles to the axis that is defined by the scanning instrument and the optical microscope. With such a probe table, the photomask can be positioned precisely relative to the two instruments.
In accordance with a further feature of the invention, it is advantageous if the mask table can be positioned by stepping motors. The motors make it possible to control the displacement of the probe table automatically. As such, it is possible to move exactly to predefined points on the mask surface. Stepping motors permit accurate position to fractions of micrometers.
In accordance with an added feature of the invention, the positioning device for positioning the scanning instrument and the opposing optical microscope relative to each other are implemented by displaceable mounting of the scanning instrument and/or the optical instrument. Displaceable mounting permits the probe of the scanning instrument to be found rapidly with the optical microscope.
In accordance with an additional feature of the invention, it is advantageous if the scanning instrument is a scanning probe microscope. Scanning probe microscopy permits the mask surface to be scanned with a high lateral resolution and is, therefore, suitable, in particular, for checking mask structures.
In accordance with yet another feature of the invention, the scanning probe microscope can be a scanning force microscope.
In the scan mode, as it is known, the scanning tip of the scanning force microscope is lowered in the direction of the mask surface. As soon as the tip has reached the mask surface, the cantilever begins to bend. From the points at which bending begins, an exact height profile of the photomask can be calculated.
Up In accordance with yet a further feature of the invention, the scanning instrument is a scanning electron microscope. In scanning electron microscopy, the surface of the mask is bombarded with electrons. The back-scattered secondary electrons are registered by a detector that can be moved piezomechanically. Information about the vapor-deposited mask structures can be derived from these secondary electrons. In particular, the position of the edges of the individual mask structures can be registered well by a scanning electron microscope.
In accordance with yet an added feature of the invention, the scanning instrument is a surface profilometer. Such a profilometer scans the mask surface with a small glass tube of about 200 nm diameter. From the points at which the small glass tube encounters the mask surface, the surface profile of the mask can be determined. The geometry of the small glass tube is advantageously matched to the mask structure to be scanned (700 nm xe2x80x9cmain feature sizexe2x80x9d).
In accordance with yet an additional feature of the invention, the illumination of the mask needed for the optical imaging is provided from the side facing away from the layer side. The light source is, therefore, disposed on the side of the optical microscope opposite the scanning instrument. The light that is incident from the microscope side passes through the glass substrate of the mask to the vapor-deposited surface structures, which reflect the incident light back to the optical microscope. In the microscope, the vapor-deposited structures, therefore, appear as areas of high light intensity. Conversely, the transparent areas of the mask appear dark because the illuminating light can pass through the glass substrate without hindrance at these points and is not reflected back to the optical microscope. Illuminating the mask using incident light geometry has the advantage that the light source is disposed on the side opposite the scanning instrument and, therefore, does not interfere in the operation of the scanning instrument. A further advantage of such an illuminating configuration is that the light source illuminates the probe through the transparent areas. Therefore, the brightly illuminated probe can be detected well by contrast with the dark background.
In accordance with again another feature of the invention, the illumination of the mask needed for the optical imaging is provided from the layer side of the mask. In the embodiment, the light source is disposed on the side of the scanning instrument. In the microscope image, the vapor-deposited structures of the mask appear as dark areas. By contrast, the transparent areas of the photomask appear light because the illuminating light can pass unimpeded to the optical microscope. Therefore, the advantage of illumination by transmitted light geometry is that the opaque areas actually appear as dark areas.
In accordance with again a further feature of the invention, illuminating the mask by transmitted light geometry is advantageous, in particular, when the illumination beam path includes at least one mirror. The mirror is disposed opposite the optical microscope and reflects the light incident from the light source in the direction of the optical microscope. In such a case, the light source can be fitted at some distance from the instruments. Such fitting prevents the light source from interfering in the operation of the scanning instrument.
In accordance with again an added feature of the invention, the optical microscope has a xe2x80x9clong working distancexe2x80x9d (LWD) objective. Such objectives have a large distance between the objective and focal plane. Using such an objective, the layer side of the mask and the probe of the scanning instrument can be imaged sharply through the glass substrate, even though the glass substrate is 2.5 mm to 6.3 mm thick.
In accordance with again an additional feature of the invention, the optical microscope is a phase microscope, and the mask used is a phase mask. Instead of transparent and opaque areas, a phase mask has areas with a different optical path length. In such a case, all the areas of one (chromium-free) phase mask are transparent. With phase contrast microscopy, however, phase differences of the light can be converted into brightness differences, and, as such, the areas of different optical path length can be made visible as areas of different brightness.
In accordance with still another feature of the invention, the optical microscope is connected to a camera, which supplies data to an image processing unit. The configuration makes possible automation of the quality control of photomasks. The evaluation of the optical microscope image no longer has to be carried out by a human and can be carried out by an image processing unit. Such a configuration permits automated quality assurance in semiconductor production.
In accordance with still a further feature of the invention, a comparison between the imaged detail of the mask and stored mask data is carried out in the image process unit. As a first result, statement relating to the quality of the photomask examined can be made. Furthermore, it is possible, by using the imaged mask structures and the stored mask data, to determine the position of the imaged detail relative to the entire mask. Thus, it is possible to determine at which point on the mask the two instruments are currently located. Even during dynamic changes in the lateral position of the mask relative to the two instruments, it is possible to specify the current observation position at any time.
In accordance with still an added feature of the invention, the image processing unit generates control signals for positioning the mask, which control the displacing device that laterally displaces the mask. The current position on the mask can be obtained from the analysis of the microscope image. If the position does not agree with the desired position, the image processing unit generates suitable control signals to drive the mask table. The mask table is repositioned until the predefined target position has been reached. As a result, it is possible to move to predefined mask areas completely automatically, and scanned and imaged. Such a configuration permits completely automated mask inspection.
In accordance with a concomitant feature of the invention, the displacing device that laterally displaces the mask is controlled such that various control structures and/or critical production mask structures located on the mask can be checked one after another. For example, a sequence of particularly critical areas can be defined that are to be moved to one after another and checked. Furthermore, at specific points on the mask, complicated control structures can be applied, with their checking permitting a statement about the quality of the mask.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a device and method for combining scanning and imaging methods in checking photomasks, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.